1. The Field of the Invention
Exemplary embodiments of the present invention relate to the field of vertical cavity surface emitting lasers (VCSELs) and, more specifically, to devices and methods to increase the reliability of VCSELs at higher temperatures.
2. The Relevant Technology
In the field of data transmission, one method of efficiently transporting data is through the use of fiber optics. Digital data is propagated through a fiber optic cable using light emitting diodes or lasers. Light signals allow for extremely high transmission rates and very high bandwidth capabilities. Also, light signals are resistant to electro-magnetic interferences that would otherwise interfere with electrical signals. Light signals are more secure because they do not allow portions of the signal to escape from the fiber optic cable as can occur with electrical signals in wire-based systems. Light also can be conducted over greater distances without the signal loss typically associated with electrical signals on copper wire.
Light signals are transmitted using a laser diode, a laser driver, and various other electrical circuitry. The transmitter receives electrical signals representing network or communication data and processes the electrical signals to achieve the result of modulating the network or communication data onto an optical signal generated by the laser driver and the associated laser.
To operate correctly, the laser diode of the transmitter is supplied with both a controlled direct current (DC) bias current and an alternating current (AC) modulation current. The DC bias current allows the laser diode to properly respond to the AC modulation. At a given temperature, a given DC bias current will produce a given optical power output for the laser. This bias current needs to provide an AC current that is at least above a threshold current level, which is the lowest excitation level at which laser output is dominated by stimulated emission rather than by spontaneous emission. However, for this given DC bias current, the optical power output will change as the temperature of the components changes.
Unfortunately, it is often necessary to compensate for these changes in optical power through some form of control network. A common method for doing this is to use a monitor photodiode that samples the optical output power as part of a control loop. The control loop can then adjust the DC bias current going to the laser to keep a constant amount of optical power output coming from the laser and going to the photodiode. This system is referred to as an average power control (APC) control loop. It compensates for both changes in the laser threshold current and slope efficiency.
As data speeds have increased to the 10 Gigabit/second standard, the requirement to operate the laser over an extended temperature range has also increased. These higher speeds produce additional problems. For a given laser design, the speed of the laser is proportional to the square root of the current above the threshold level. Thus, in order to get the most speed from the laser, it is desirable to drive the laser with a large amount of current. Unfortunately, the reliability of the laser decreases proportionally to the square of the current. Additionally, the reliability of the laser decreases exponentially as the temperature increases.
The above described relationships are shown graphically in FIG. 1, which is designated generally as reference numeral 10. Graph 10 includes a relative current scale 12 on the bottom. Relative current is defined as the current applied to the laser, I, minus the threshold current, Ith, divided by Ith. The right side of the graph shows the resonance frequency of the laser 14, which corresponds to the speed of the laser. The left hand side of the graph shows the Mean Time to Failure (MTTF) 16 in hours.
A first plot line 18 illustrates the relationship between current and laser speed. As the current increases, the speed of the laser increases as well. Lines 20, 22, and 24 represent the relationship between speed and reliability at temperatures of 0, 40 and 80° C., respectively. Note that, as the temperature and current increase, the reliability of the laser decreases significantly.
This relationship is somewhat more complicated for vertical cavity surface emitting lasers (VCSELs). Vertical cavity surface emitting lasers (VCSELs) represent a relatively new class of semiconductor lasers. While there are many VCSEL variations, a common characteristic is that VCSELs emit light perpendicular to a semiconductor wafer's surface. Advantageously, VCSELs can be formed from a wide range of material systems to produce specific characteristics.
VCSELs include, among other things, semiconductor active regions, distributed Bragg reflector (DBR) mirrors, current confinement structures, substrates, and contacts. Because of their complicated structure, and because of their specific material requirements, VCSELs are usually grown using metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). Specific details concerning the construction of a typical VCSEL are known to those of skill in the art. However, for the sake of clarity, a brief discussion of a typical VCSEL structure is provided below with reference to FIG. 2.
FIG. 2 illustrates a typical long-wavelength VCSEL 110. As shown, an n-doped Indium Phosphide (InP) substrate 112 has an n-type electrical contact 114. An n-doped lower mirror stack 116 (a Distributed Bragg Reflector (DBR)) is on the InP substrate 112, and an n-type graded-index InP lower spacer 118 is disposed over the lower mirror stack 116. An indium gallium arsenide phosphide (InGaAsP) or aluminum indium gallium arsenide (AlInGaAs) active region 120, usually having a number of quantum wells, is formed over the InP lower spacer 118. Over the active region 120 is an insulating region 140 that provides current confinement. The insulating region 140 is usually formed either by implanting protons or by forming an oxide layer. In any event, the insulating region 140 defines a conductive annular central opening 142 that forms an electrically conductive path though the insulating region 140. Over the insulating region is a tunnel junction 128. Over the tunnel junction 128 is an n-type graded-index InP top spacer 122 and an n-type InP top mirror stack 124 (another DBR), which is disposed over the InP top spacer 122. Over the top mirror stack 124 is an n-type conduction layer 109, an n-type cap layer 108, and an n-type electrical contact 126.
Still referring to FIG. 2, the lower spacer 118 and the top spacer 122 separate the lower mirror stack 116 from the top mirror stack 124 such that an optical cavity is formed. As the optical cavity is resonant at specific wavelengths, the mirror separation is controlled to resonate at a predetermined wavelength (or at a multiple thereof).
In operation, the external bias current discussed above causes an AC electrical current 121 to flow from the electrical contact 126 toward the electrical contact 114. The tunnel junction over the insulating region 140 converts incoming electrons into holes. The converted holes are injected into the insulating region 140 and the conductive central opening 142, both of which confine the current 121 such that the current flows through the conductive central opening 142 and into the active region 120. Some of the injected holes are converted into photons in the active region 120. Those photons bounce back and forth (resonate) between the lower mirror stack 116 and the top mirror stack 124. While the lower mirror stack 116 and the top mirror stack 124 are very good reflectors, some of the photons leak out as light 123 that travels along an optical path. The light 123 passes through the conduction layer 109, the cap layer 108, an aperture 130 in electrical contact 126, and out of the surface of the vertical cavity surface emitting laser 110.
It should be understood that FIG. 2 illustrates a typical long-wavelength VCSEL having a tunnel junction, and that numerous variations are possible. For example, the dopings can be changed (say, by providing a p-type substrate), different material systems can be used, operational details can be tuned for maximum performance, and additional structures and features can be added.
In a typical VCSEL, such as the one discussed above, the relationship between the applied bias current and the speed of the laser is more complex due to the multi-transverse mode nature of the device. Generally, one can observe differences in the modulation characteristics as a function of temperature, and more specifically, depending on which side of the “T-zero” (T0) point the laser is operated. The T0 point is the temperature at which the threshold current is a minimum over temperature. Also, the slope efficiency (amount of light output increase for a given increase in drive current) can affect the modulation performance.
FIG. 3 illustrates one example, designated generally as reference numeral 30, of the variation of threshold current 32 and slope efficiency 34 over temperature 36 in a typical VCSEL. A first plot line 38 represents the variation of threshold current as a function of temperature. A second plot line 40 represents the variation of slope efficiency over temperature. As can be seen from the plot line 40, in a VCSEL, the slope efficiency decreases with increasing temperature.
When operated in a constant optical power mode, the VCSEL 100 described here would have the operational characteristics shown in a graph 50 illustrated in FIG. 4. The graph 50 includes a temperature scale 52, a relative laser speed 54, and a measure of the bias current 56. A first curve 58 illustrates the relative speed of the laser as the temperature increases. A second curve 60 illustrates the bias current needed to maintain a constant power output as the temperature increases. As can be clearly seen in curve 58, as the temperature decreases, the optical performance is degraded due to a reduction in speed. Similarly, at high temperature, the reliability of the VCSEL is compromised due to the increase in the bias current, as shown in curve 60. As can be seen from the graph in FIG. 4, using an APC control loop for VCSEL 100 requires a very high bias current at higher speeds and higher temperatures. Unfortunately, at these higher temperatures, the reliability of the VCSEL is severely compromised due to the increase in the bias current.